von Willebrand factor (VWF) is a multimeric glycoprotein in plasma that plays an important role in hemostasis by mediating platelet binding to sites of vascular injury. In recent studies, VWF has also been implicated in microvascular dysfunction and occlusion, in part because of its unique ability to sel?associate and form hyper-adhesive strands of enormous sizes attached to the endothelial surface in response to hydrodynamic forces, including shear stress and elongation flow. When these strands are not removed by the metalloprotease ADAMTS13 in plasma, they bind platelets efficiently, and the accumulation of VWF?platelet thrombi leads to vessel occlusion, tissue infarction, and organ dysfunction. We recently discovered that high density lipoprotein (HDL), a well?known cardioprotective lipoprotein in plasma, and its major component protein apolipoprotein (Apo)A?I, can attenuate the extent of VWF self?association, and ultimately the severity of thrombotic complications in the vasculature. These studies unveiled a novel antithrombotic property of HDL/ApoA?I, which we hypothesize is very important in diseases characterized by microvascular occlusion. In this application, we will focus on the mechanism of VWF self?association, how VWF interacts with HDL/ApoA?I, and the physiologic impact of the HDL-VWF interaction. In Specific Aim 1, we will identify the VWF self? association site exposed by hydrodynamic forces by peptide mapping and use of VWF variants. In Specific Aim 2, we will determine the effect of VWF self?association on ADAMTS13?mediated cleavage under shear stress, and assess the role of HDL in ADAMTS13?mediated cleavage of VWF. In Specific Aim 3, we will evaluate in mouse models of TTP and sepsis whether the outcome and disease parameters are worsened by HDL deficiency or improved by HDL treatment. Successful completion of these aims will provide an improved understanding of the basic mechanism of VWF self?association, how this process can be regulated, and how this information can be used to develop new approaches to treat systemic diseases caused by dysregulation of VWF self?association.